"Although I'm relatively new to using AskUbuntu..."
Welcome! I'll attempt to answer your questions in no particular order.
"BTW, I found no appropriate tag for this questions, so I hope it's not OT..."
This is definitely the correct tag for your question.
"I have, at times, wanted to contact the "leaders" responsible for the direction, decisions and development of AskUbuntu (ie. who decides the "official rules"?)."
Actually, the "leaders" are the Ask Ubuntu users themselves. In fact, this site (meta.askubuntu.com) is where a lot of the discussion regarding the direction of the website takes place.
Flagging, closing, and editing can be done by any user with sufficient privileges. Only when issues escalate beyond what the community can deal with on its own are the site moderators brought in. If the issue goes beyond what they can deal with (which is extremely rare), someone from the Stack Exchange team steps in.
"Who are these individuals, and how do I contact them?"
The moderators can be contacted by flagging the item in question and alerting them. If the issue involves a user, flag one of their posts. The Stack Exchange team can by contacted by following the link you included in your question.
"(For instance, I don't want my message to be made public.)"
The moderators have all agreed to the moderator agreement, of which one of the clauses states:
"I will not disclose this information to anyone,"
Therefore, you can be certain that any communication will be kept confidential.